1. Field of the Invention
This invention relates to a fire alarm system and, more particularly, to a fire alarm system including a receiver having the function of detecting a fire based on analog detection data contained in a received signal, at least one analog type sensor provided in at least one monitored area monitored by the receiver, in combination with one or more on-off type sensor, and a transmission line interconnecting the analog type and on-off type sensors and the receiver.
2. Description of the Prior Art
Such system the fire alarm system, such system in which a plurality of so-called on-off type sensors each having a switching element which is turned on in case of fire detection are connected to a transmission line from a receiver, is most popular. This on-off type sensor senses changes in the surrounding physical phenomena caused by the fire, such as those in the temperature or smoke concentration. When the magnitude of the changes exceeds a predetermined threshold value, the switching element is actuated to transmit fire alarm signals or, more precisely, current signals, to the receiver over the transmission line. In short, with the on-off type sensor, it is the sensor itself that decides whether or not the fire has broken out.
On the other hand, a fire alarm system has been evolved in which it is not the sensor itself, as in the on-off type sensor, but the receiver, that has the function of determining whether or not the fire has broken out. In this case, an analog type sensor, which senses the extent of the changes in the surrounding physical phenomena caused by the fire as an analog value, is used as the sensor. The analog detection signal outputted from the sensor is transmitted over the transmission line to the receiver where it is determined from the received analog detection data whether or not the fire has broken out.
With such analog fire alarm system, since the receiver gives a judgment as to the possible occurrence of a fire, it becomes possible to perform ingenious signal processing, such as predictive judgment on the fire occurrence. Also, since the analog data itself is handled, a mistaken fire report, such as is often made by the on-off type sensor, cannot be made, so that early fire detection can be achieved with the minimum risk of mistaken signalling.
With such analog fire alarm system, it becomes necessary to distinguish output analog data from a plurality of analog type sensors connected to the same transmission network. For this reason, an address is set for each of the analog type sensors and each sensor is adapted to sequentially transmit its own analog detection data to the receiver over the transmission line by time divisional multiplexed transmission according to a polling system. In this case, a reset pulse and a plurality of clock pulses are transmitted at a predetermined time interval from the receiver to the analog type sensors. The clock pulses start to be counted at the sensor from the time when the reset pulse is received. When the count value reaches the clock count allocated to a specific receiver, it is determined that the receiver has been interrogated, and the analog detection data contained in the receiver at this time is sent to the receiver after conversion into a corresponding electrical current value.
However, in such conventional analog fire alarm system, since the sensors are sequentially interrogated by relying upon the addresses allocated thereto, the time period involved in interrogating the sensors, that is the polling period, is necessarily increased with the increase in the number of the sensors. The result is that limitations are imposed on the number of the sensors that can be interrogated within a predetermined time period within the allowable limit of the delay in fire detection and hence on the number of the sensors that can be provided in each network.
Thus, in a monitor area of a larger spatial capacity, such as a large hall, it occurs frequently that the number of the sensors to be installed exceeds the allowable limit per network, with the result that plural analog sensor networks need be installed in one and the same monitor area by means of receivers or relays with complicated layout and increased costs.
On the other hand, provision of an address sensor in one monitor area results in increased costs and a complicated control operation at the receiver on fire occurrence with a corresponding broad application to the control section.